2.1. Conditioned Cell Media
Culture medium compositions typically include essential amino acids, salts, vitamins, minerals, trace metals, sugars, lipids and nucleosides. Cell culture medium attempts to supply the components necessary to meet the nutritional needs required to grow cells in a controlled, artificial and in vitro environment. Nutrient formulations, pH, and osmolarity vary in accordance with parameters such as cell type, cell density, and the culture system employed. Many cell culture medium formulations are documented in the literature and a number of media are commercially available. Once the culture medium is incubated with cells, it is known to those skilled in the art as “spent” or “conditioned medium”. Conditioned medium contains many of the original components of the medium, as well as a variety of cellular metabolites and secreted proteins, including, for example, biologically active growth factors, inflammatory mediators and other extracellular proteins. Cell lines grown as a monolayer or on beads, as opposed to cells grown in three-dimensions, lack the cell-cell and cell-matrix interactions characteristic of whole tissue in vivo. Consequently, such cells secrete a variety of cellular metabolites although they do not necessarily secrete these metabolites and secreted proteins at levels that approach physiological levels. Conventional conditioned cell culture medium, medium cultured by cell-lines grown as a monolayer or on beads, is usually discarded or occasionally used in culture manipulations such as reducing cell densities.
2.2. Tissue Culture Systems
The majority of vertebrate cell cultures in vitro are grown as monolayers on an artificial substrate bathed in culture medium. The nature of the substrate on which the monolayers grow may be solid, such as plastic, or semisolid gels, such as collagen or agar. Disposable plastics have become the preferred substrate used in modern-day tissue or cell culture.
A few researchers have explored the use of natural substrates related to basement membrane components. Basement membranes comprise a mixture of glycoproteins and proteoglycans that surround most cells in vivo. For example, Reid and Rojkund, 1979, In, Methods in Enzymology, Vol. 57, Cell Culture, Jakoby & Pasten, eds., New York, Acad. Press, pp. 263-278; Vlodavsky et al., 1980, Cell 19:607-617; Yang et al., 1979, Proc. Natl. Acad. Sci. USA 76:3401 have used collagen for culturing hepatocytes, epithelial cells and endothelial tissue. Growth of cells on floating collagen (Michalopoulos and Pitot, 1975, Fed. Proc. 34:826) and cellulose nitrate membranes (Savage and Bonney, 1978, Exp. Cell Res. 114:307-315) have been used in attempts to promote terminal differentiation. However, prolonged cellular regeneration and the culture of such tissues in such systems has not heretofore been achieved.
Cultures of mouse embryo fibroblasts have been used to enhance growth of cells, particularly at low densities. This effect is thought to be due partly to supplementation of the medium but may also be due to conditioning of the substrate by cell products. In these systems, feeder layers of fibroblasts are grown as confluent monolayers which make the surface suitable for attachment of other cells. For example, the growth of glioma on confluent feeder layers of normal fetal intestine has been reported (Lindsay, 1979, Nature 228:80).
While the growth of cells in two dimensions is a convenient method for preparing, observing and studying cells in culture, allowing a high rate of cell proliferation, it lacks characteristic of whole tissue in vivo. In order to study such functional and morphological interactions, a few investigators have explored the use of three-dimensional substrates such as collagen gel (Douglas et al., 1980, In Vitro 16:306-312; Yang et al., 1979, Proc. Natl. Acad. Sci. 76:3401; Yang et al., 1980, Proc. Natl. Acad. Sci. 77:2088-2092; Yang et al., 1981, Cancer Res. 41:1021-1027); cellulose sponge, alone (Leighton et al., 1951, J. Natl. Cancer Inst. 12:545-561) or collagen coated (Leighton et al., 1968, Cancer Res. 28:286-296); a gelatin sponge, Gelfoam (Sorour et al., 1975, J. Neurosurg. 43:742-749).
In general, these three-dimensional substrates are inoculated with the cells to be cultured. Many of the cell types have been reported to penetrate the matrix and establish a “tissue-like” histology. For example, three-dimensional collagen gels have been utilized to culture breast epithelium (Yang et al., 1981, Cancer Res. 41:1021-1027) and sympathetic neurons (Ebendal, 1976, Exp. Cell Res. 98:159-169). Additionally, various attempts have been made to regenerate tissue-like architecture from dispersed monolayer cultures. (Kruse and Miedema, 1965, J. Cell Biol. 27:273) reported that perfused monolayers could grow to more than ten cells deep and organoid structures can develop in multilayered cultures if kept supplied with appropriate medium (see also Schneider et al., 1963, Exp. Cell. Res. 30; 449-459; Bell et al., 1979, Proc. Natl. Acad. Sci. USA 76:1274-1279; Green, 1978, Science 200:1385-1388). It has been reported that human epidermal keratinocytes may form dematoglyphs (friction ridges if kept for several weeks without transfer; Folkman and Haudenschild (1980, Nature 288:551-556) reported the formation of capillary tubules in cultures of vascular endothelial cells cultured in the presence of endothelial growth factor and medium conditioned by tumor cells; and Sirica et al. (1979, Proc. Natl. Acad. Sci. USA 76:283-287; 1980, Cancer Res. 40:3259-3267) maintained hepatocytes in primary culture for about 10-13 days on nylon meshes coated with a thin layer of collagen. However, the long term culture and proliferation of cells in such systems has not been achieved.
The establishment of long term culture of tissues such as bone marrow has been attempted. Overall the results were disappointing, in that although a stromal cell layer containing different cell types is rapidly formed, significant hematopoiesis could not be maintained for any real time. (For review see Dexter et al., In Long Term Bone Marrow Culture, 1984, Alan R. Liss, Inc., pp. 57-96).
A number of groups have attempted to grow skin and connective tissue in vitro for transplantation in vivo. In one such system, a hydrated bovine collagen lattice forms the substrate to which cells, such as fibroblasts are incorporated which results in the contraction of the lattice into tissue (Bell et al., U.S. Pat. No. 4,485,096). In another system, a porous cross-linked collagen sponge is used to culture fibroblast cells (Eisenberg, WO 91/16010). A scaffold composed of synthetic polymers has also been described to control cell growth and proliferation in vitro so that once the fibroblasts begin to grow and attach to the matrix it is transplanted into the patient (Vacanti et al., U.S. Pat. Nos. 5,759,830; 5,770,193; 5,736,372).
Synthetic matrices composed of biodegradable, biocompatible copolymers of polyesters and amino acids have also been designed as scaffolding for cell growth (U.S. Pat. Nos. 5,654,381; 5,709,854). Non-biodegradable scaffolds are likewise capable of supporting cell growth. Three-dimensional cell culture systems have also been designed which are composed of a stromal matrix which supports the growth of cells from any desired tissue into an adult tissue (Naughton et al., U.S. Pat. Nos. 4,721,096 and 5,032,508). Another approach involves slowly polymerizing hydrogels containing large numbers of the desired cell type which harden into a matrix once administered to a patient (U.S. Pat. No. 5,709,854). Extracellular matrix preparations have been designed which are composed of stromal cells which provide a three dimensional cell culture system for a desired cell type which may be injected into the patient for precise placement of the biomaterial (Naughton et al., WO 96/39101).
2.3. Cellular Cytokines and Growth Factors
The secretion of extracellular proteins into conditioned cell media such as growth factors, cytokines, and stress proteins opens new possibilities in the preparation of products for use in a large variety of areas including tissue repair, e.g., in the treatment of wounds and other tissue defects such as cosmetic defects as well as human and animal feed supplements. For example, growth factors are known to play an important role in the wound healing process. In general, it is thought desirable in the treatment of wounds to enhance the supply of growth factors by direct addition of these factors.
Cellular cytokines and growth factors are involved in a number of critical cellular processes including cell proliferation, adhesion, morphologic appearance, differentiation, migration, inflammatory responses, angiogenesis, and cell death. Studies have demonstrated that hypoxic stress and injury to cells induce responses including increased levels of mRNA and proteins corresponding to growth factors such as PDGF (platelet-derived growth factor), VEGF (vascular endothelial growth factor), FGF (fibroblast growth factor), and IGF (insulin-like growth factor) (Gonzalez-Rubio, M. et al., 1996, Kidney Int. 50(1):164-73; Abramovitch, R. et al., 1997, Int J. Exp. Pathol. 78(2):57-70; Stein, I. et al. 1995, Mol Cell Biol. 15(10):5363-8; Yang, W. et al., 1997, FEBS Lett. 403(2):139-42; West, N. R. et al., 1995, J. Neurosci. Res. 40(5):647-59).
Growth factors, such as transforming growth factor-β, also known in the art as TGF-β, are induced by certain stress proteins during wound healing. Two known stress proteins are GRP78 and HSP90. These proteins stabilize cellular structures and render the cells resistant to adverse conditions. The TGF-β family of dimeric proteins includes TGF-β1, TGF-β2, and TGF-β and regulates the growth and differentiation of many cell types. Furthermore, this family of proteins exhibits a range of biological effects, stimulating the growth of some cell types (Noda et al., 1989, Endocrinology 124:2991-2995) and inhibiting the growth of other cell types (Goey et al., 1989, J. Immunol. 143:877-880; Pietenpol et al., 1990, Proc. Natl. Acad. Sci. USA 87:3758-3762). TGF-β has also been shown to increase the expression of extracellular matrix proteins including collagen and fibronectin (Ignotz et al., 1986, J. Biol. Chem. 261:4337-4345) and to accelerate the healing of wounds (Mustoe et al. 1987, Science 237:1333-1335).
Another such growth factor is PDGF, PDGF was originally found to be a potent mitogen for mesenchymal-derived cells (Ross R. et al., 1974, Proc. Natl. Acad. Sci. USA 71(4):1207-1210; Kohler N. et al., 1974, Exp. Cell Res. 87:297-301). Further studies have shown that PDGF increases the rate of cellularity and granulation in tissue formation. Wounds treated with PDGF have the appearance of an early stage inflammatory response including an increase in neutrophils and macrophage cell types at the wound site. These wounds also show enhanced fibroblast function (Pierce, G. F. et al. 1988, J. Exp. Med. 167:974-987). Both PDGF and TGF-β have been shown to increase collagen formation, DNA content, and protein levels in animal studies (Grotendorst, G. R. et al., 1985, J. Clin. Invest. 76:2323-2329; Sporn, M. B. et al., 1983, Scienice (Wash D.C.) 219:1329). PDGF has been shown to be effective in the treatment of human wounds. In human wounds, PDGF-AA expression is increased within pressure ulcers undergoing healing. The increase of PDGF-AA corresponds to an increase in activated fibroblasts, extracellular matrix deposition, and active vascularization of the wound. Furthermore, such an increase in PDGF-AA is not seen in chronic non-healing wounds (Principles of Tissue Engineering, R. Lanza et al. (eds.), pp. 133-141 (R.G. Landes Co. TX 1997). A number of other growth factors having the ability to induce angiogenesis and wound healing include VEGF, KGF and basic FGF.
There are currently no simple effective methods or compositions for application containing the variety of cytokines, growth factors or other regulatory proteins found in Applicants' conditioned media.